What version of ZoneFlex software are you running?
What model of AP is it?
Have you tried to factory reset that AP? If not, try that.
Is the AP directly connected to the ZoneDirector, or is it going through a routed network?

It's possible the AP is just failing, so if it's critical to your network I would recommend you open a support case.

Have you tried to factory reset that AP? If not, try that. no, it is in production environmenet, i will do it like a last solution only

Is the AP directly connected to the ZoneDirector, or is it going through a routed network? All APs are in each floor of the building, connected directly to the floor switch. All floor switch are connected to the data center switch so it is like all APs and Zone Director are connected to the same switch

No, I don't think an upgrade will do anything*. From your further description, the issue is either specific to that AP or the ethernet segment it is on. When you can plan some downtime, swapping another AP in it's place should tell you which is the problem (if the new AP works..it's the other AP). (Pick an AP from a less critical location if you can, so that if you do end up having to RMA the unit it won't be so painful)

*There is one situation where very high PHY errors can cause the AP to re-boot. This would suggest a lot of noise in the area around that AP. More recent firmware such as 9.5.3 handles this better.

Further diagnosis will probably require that you open a support case, so that's my best advise..